Adaptive switched filter arrangement for use in rapid frequency tracking

ABSTRACT

An adaptive switched filter arrangement formed from two switched filters to each of which is applied the same input signal. The first switched filter is switched at an independent predetermined frequency. The second switched filter, however, is tuned by the output frequency extracted by the first filter. This minimizes phase tracking time and cancels systematic phase error.

United States Patent Lautier et al. [451 June 6, 1972 [54] ADAPTIVESWITCHED FILTER [56] References Cited ARRANGEMENT FOR USE IN RAPIDFREQUENCY TRACKING UNITED STATES PATENTS Inventors: Alex more Laufier,vence; Jean Louis 3,403,345 9/ 1968 Frank etal ..333/70A X 2:22aTourenes Sur Loup both of Primary Examiner-Paul L. GenslerAttorney-Hanifin & Jancin and Robert Bruce Brodie [73] Assignee:International Business Machines Corporation, Armonk, NY. [57] ABSTRACTFiledi 1970 An adaptive switched filter arrangement formed from two J 7switched filters to each of which is applied the same input L21] No 9530signal. The first switched filter is switched at an independentpredetermined frequency. The second switched filter, how- [52] US. Cl...333/70 A, 328/155, 328/167 ever, i ned by the out ut frequencyextracted by the first filter. This minimizes phase tracking time andcancels syste- [58] Field of Search ..333/17, 70, 70 A; 328/138, maticphase emm SWF 1 BPF 1 6 Claim, 4 Drawlng Figures SWF 2 BPF 2 PATENTEDJUHs [an 3, 668, 570

FIG. 1

PRIOR ART ER SWF1 BPFi FIG. 2 H1 SWF 2 BPF 2 SHIFT 4H FIG. 3

SH|FT=2Hz NON SHIFT 30 4b 50 FIG. 4 10 I i% 5 16 2 0 5b 4B 50 INVENTORSALEX H. LAUTIER JEAN L. HONRO IN BYM ZQ ATTORNE Y ADAPTIVE SWITCI'IEDFILTER ARRANGEMENT FOR USE IN RAPID FREQUENCY TRACKING BACKGROUND OF THEINVENTION This invention relates to an arrangement of narrow bandpassfiltering devices and more particularly to an arrangement of N-path typefilters or switched filters for use in, for example, rapid frequencytracking.

Switched filters are known in the prior art as tunable very high Qband-pass filters of the active circuit type. These are described, forexample, in Electronics, July 24, 1967 at pages 90-100 in an articleentitled Digital Filters with lC's Boost Q Without Inductors" by WilliamR. Hardin. Also the principles are set forth in Electrical Design News,June 1, 1967, published by Rogers Publishing Company of Denver, Coloradoin A Novel Approach to Wave Filters" by Christopher Vale. Lastly, anelaborate mathematical exposition is revealed in the Bell SystemTechnical Journal, pages l,32ll,350, September, 1960 in An AlternativeApproach to the Realization of Network Transfer Functions by L. E.Franks et al.

In the Hardin reference, a simple shunt switched filter is shown formedby connecting several capacitors in parallel to segments of a rotaryswitch. The circuit is returned to ground through the switch. An outputvoltage is developed across the capacitor and switch segment grounded atany one instant of time. An applied time varying input voltage that isnot synchronized with the speed of rotation of the switch f, willdevelop a voltage across the switched capacitors averaging out to zero.However, any input signal having the same period of repetition as toswitch wiper will charge up the capacitors and thus yield an output.Given identical capacitors in such a switch it can be shown, that thefilter has a bandwidth of Af= l/RC centered as 1",, with a QEfl/Af.

As pointed out in the Vale reference, a simple bandpass filter can beconstructed from three capacitors. In the first case where capacitance C0.1 pf was used, a Q= 35 was obtained for a switching frequency f; 1,250Hz and a bandwidth Af= 35 Hz. Af was measured between the 3 db points onthe relative magnitude-frequency characteristic of the device. Uponcapacitance C being increased to 0.47 pf, a Q= 166 was obtained for thesame switching frequency f, 1,250 Hz and a Af= 7.5 Hz. Extremely highQ's can also be obtained if the filter switching speed is substantiallyincreased, the shape of the filter characteristic remaining constant.However, switched filters generally induce a phase shift between theinput and output signals due to the inevitable difference between thenominal frequency of the filter and the input signal.

It is accordingly, an object of the present invention to provide aswitched filter device delivering an output signal with a phase shiftequal to zero with respect to the incident signal in steady state.

It is yet another object of the present invention to provide a deviceenabling simple and quick recovery of a given frequency with a minimumphase error in transient state.

It is still further another object of the present invention to providean adaptive device enabling frequency tracking.

SUMMARY OF THE INVENTION The present invention comprises two switchedfilters, both receiving the incident signal at their respective inputs.The first switched filter is switched by a fixed frequency source closeto the frequency to be extracted. The second switched filter iscontrolled by the frequency actually extracted by the first switchedfilter.

The output signal of the first filter presents a systematic phase error45 proportional to the frequency shift between the input signal and thecontrol signal. The phase shift involves, at the generator associated tothe second switched filter, a frequency shift between the signalreceived by the second filter, which is the incident signal, and thesignal controlling this second filter, this frequency shift beingproportional to the derivative of phase shift The frequency controllingthe first filter being fixed in steady state, phase shift is constantand its derivative is null. Thus, the second filter is tuned on thefrequency received and its output signal does not show any systematicphase shift.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF SUMMARY OF THE DRAWING FIG. 1 shows a standard shunt switchedfilter. FIG. 2 shows an embodiment of the present invention. FIGS. 3 and4 show the phase shift variation in the case of a standard switchedfilter and in the case of the device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The standard switched filtershown in FIG. 1 comprises a resistor R, one temiinal of which receivesthe incident signal and the other terminal of which is connected, toseveral capacitors. The values of the capacitors may be different. Theresistor may also be coupled to the input of a BPFl standard band-passfilter. For simplicity, this shunt type switched filter is limited tofour capacitors with the same value C. These capacitors are grounded inturn through switches I following a cycle determined by generator H1controlled by a fixed pulse signal w The operation of such a filter isdescribed in detail in the previously named Hardin and Vale references.The following mathematical analysis is intended to demonstrate thesystemic phase shift error in this type of filter. Let us apply at thefilter input a unit step cosine signal x(t) such that x(t) Y(t) cos (wt(1)) where Y(t) is the Heaviside function or the unit step.

a: is the signal pulse repetition frequency in radians per second.

o is the constant phase shift introduced by the transmission.

The transfer function of the switched filter may be represented as ecosm t Mirwhere 0 NRC N is the number of capacitors The output signal Y(t)of such a filter is given by the following formula:

m x(t) @F whereB convolution operator Thus where m and g are generalizedfunctions:

+ 112] e cos (n h) dlt iicuH-dt) 1 1 Up to now, we have considered thesteady state only; let us 2 consider the phenomena in transient statenow.

At point A, the pulsation of the signal delivered by filter SWFI willtend toward pulsation m which is obtained in steady .2. 1 Therefore,SWF2 is a switched filter, the incident signal of lwwr 1 1 5 state atthis same point.

i; j ij which has (0 for pulsation, since this filter directly receivesthe 6 incident signal and the signal of which controlling thegenerawhich can be represented in the following form: tor associated tothis same filter has a pulsation which tends P=Kcos(wt+d +l) I (2) whereK=constant FIGS. 3 and 4 show the phase responses at the output of lConstant phase shift due to the frequency shift filter SWFI and at theoutput of filter SWF2 according to the between the incident signal andpulse control signal (0 7 time and shift between the frequency of theincident signal we and a frequency of 2,800 Hz controlling generator H1.3) T i 4,) The use of two switched filters enables cancellation of the i(mo y phase error to first order; with the same principle, the use of anadditional switched filter controlled by the frequency isolated byfilter SWFZ and receiving the incident signal at its 1 /0 6 605 2 2O:51; M11 permit the cancellation of the phase error to second & (moTherefore, the total number of switched filters used will depend on theaccuracy required.

It is to be noted that the great adaptability of this device 1 Sm (Wt(1)) 5 cos (0)": an] enables tracking of the incident signal frequencywith a great selectivity. in fact, if we determine a certain bandwidthfor if (JD-tw to first order, this expression can be represented in filtsw and a narrower bandwidth f filt SWFZ the the followmg form: latterbeing permanently tuned on the incident signal frequency, will enabletracking of this frequency within the range (4) T 5* 0e" COS (w t 4 3Odefined by filter SWFl.

It is to be noted that although we have used a switched filter as firstfilter (SWFI) in this invention, any other filtering Referring now toFIG. 2, there is an example of a quick d i giving a systematic phaseshift such as phase locked response switched filter in accordance withthe present invenoscillator could be used as the first filten tion. Thisdevice comprises a first standard switched filter SWFl controlled by apulse generator H1 controlled by a fixed pulsation signal to, close tothe pulsation of the signal to be extracted, followed by a standardband-pass filter BPFI. The output of filter BPH is connected to theinput of pulse generator H2 associated to switched filter SWFZ, thefrequency controlling this generator H2 being the frequency actuallyextracted by filter SWFl. The input of filter SWF2 receives the incidentsignal and its output is connected to a second standard band-pass filterBPFZ the output of which is the output of the device.

Let use suppose that the input signal of the device is in the followingform:

5 A such device will be advantageously used in data transmissions eitherfor the correct setting of a clock or for the recovery of carrierfrequencies on other similar systems.

This description of the present invention has been given as an exampleand it will be understood that various changes in 0 form and details maybe made therein without depart from the spirit and scope of theinvention.

What is claimed is: 1. A filter arrangement for recovering therepetition frequency w of a communication signal comprising:

a phase locked oscillator of frequency (n responsive to thecommunication signal for producing a signal of frequency to, when w iswithin the oscillator frequency passband;

e= Y(t) cos (all +45) and z fgajgfig g zigfgf i a f sa ifji z ig l s gza switchable filter also responsive to the communications y g g y psignal and being switched at the (0 rate by the oscillator shift l toappear at the output of filter SWFl in accordance with the indicationgiven before.

Thus, at point A, we have a signal of the following form.

X Kcos (mt To first order, 1 can be represented in the following form:

output for producing an output when m lies within the filter frequencypassband.

A filter arrangement according to claim 1, wherein:

a switchable filter defines a frequency band-pass characteristic wherew, is the center frequency, w, 1/1- is the (5) (bl o(w w duo F (61)constant upper limit, w, 1/1 is the lower limit, and 1- is the filtertime constant. 01 0 particular of filter S W F l 3. A filter arrangementaccording to claim 1 wherein: The phase Shift #1 causes a generator H2Shift between the bandwidth of the switchable filter is less than thephase locked oscillator.

the incident frequency and the frequency controlling generator H2. if012 is the pulsation of the signal controlling genera- A filtermangemfim Fecovering fi repetition tor H2, we will hav frequency w of acommunications signal comprising:

m2 m (dos 1 Mt) 6) a source of clock signals of frequency (0,;

a first switchable filter responsive to the communications In Formula(5) above, qbo, m and m being constant, phase I 65 signal, said filterbeing switched by the clock source at the shift d l is constant, andfrom Formula (6), we have:

02 w 0),, rate and defining a frequency band-pass characteristic At filtwpz if we transpose Formula (5) at (62 phase where w is the centerfrequency, to l/r is the upper hift produced by u :2 limit, 0),, l/r, isthe lower limit, and r, is the filter time constant, said filter furtherproducing a signal frequency (b2 '0(w m2) 0 Q5 (0) F (62) constant alwhen 1 lies within the first filter frequency passband;

and 62 0 7 a second switchable filter responsive to the communicationsTo first order, therefore the device extracts the desired Signal Saidfilter beingiswitched by the first filter Outpm frequency withoutintroducing systematic phase shift in steady at the l fate and definmg afrequency -W characsmte teristic where w, is the center frequency, w,1/7 is the pp limit, i 1/72 i the lower im n 1'2 i h filter carriersignal of frequency w, e and switchable by the time constant, forproducing an output when wlies within lo k signal source at the a), ratefor producing a the second filter frequency passband. sinusoidal signaloutput of m, e being the first 5. A filter arrangement according toclaim 4 wherein: filter phase hift; and

r 1' 5 a second switchable filter responsive to the shifted carrier 6. Afilter arrangement for recovering sinusoidal carrier signal switchableat the (a, e d: rate by the first filter signal of frequency w asshifted by frequency of e, comprising: output for passing the m 6signal.

a source of clock signals of frequency m a first switchable filterresponsive to the shifted sinusoidal

1. A filter arrangement for recovering the repetition frequency omega ofa communication signal comprising: a phase locked oscillator offrequency omega o responsive to the communication signal for producing asignal of frequency omega 1 when omega is within the oscillatorfrequency passband; and a switchable filter also responsive to thecommunications signal and being switched at the omega 1 rate by theoscillator output for producing an output when omega lies within thefilter frequency passband.
 2. A filter arrangement according to claim 1,wherein: a switchable filter defines a frequency band-passcharacteristic where omega 1 is the center frequency, omega 1 + 1/ Tauis the upper limit, omega 1 - 1/ Tau is the lower limit, and Tau is thefilter time constant.
 3. A filter arrangement according to claim 1wherein: the bandwidth of the switchable filter is less than the phaselocked oscillator.
 4. A filter arrangement for recovering the repetitionfrequency omega of a communications signal comprising: a source of clocksignals of frequency omega o; a first switchable filter responsive tothe communications signal, said filter being switched by the clocksource at the omega o rate and defining a frequency band-passcharacteristic where omega o is the center frequency, omega o + 1/ Tau 1is the upper limit, omega o - 1/ Tau 1 is the lower limit, and Tau 1 isthe filter time constant, said filter further producing a signalfrequency omega 1 when omega lies within the first filter frequencypassband; and a second switchable filter responsive to thecommunications signal, said filter being switched by the first filteroutput at the omega 1 rate and defining a frequency band-passcharacteristic where omega 1 is the center frequency, omega 1 + 1/ Tau 2is the upper limit, omega 1 - 1/ Tau 2 is the lower limit; and Tau 2 isthe filter time constant, for producing an output when omega lies withinthe second filter frequency passband.
 5. A filter arrangement accordingto claim 4 wherein: Tau 1 < Tau
 2. 6. A filter arrangement forrecovering sinusoidal carrier signal of frequency omega o as shifted byfrequency of epsilon , comprising: a source of clock signals offrequency omega o; a first switchable filter responsive to the shiftedsinusoidal carrier signal of frequency omega o + epsilon and switchableby the clock signal source at the omega o rate for producing asinusoidal signal output of omega o + epsilon + phi , phi being thefirst filter phase shift; and a second switchable filter responsive tothe shifted carrier signal switchable at the omega o + epsilon + phirate by the first fiLter output for passing the omega o + epsilonsignal.